NEW MEXICO BUREAU OF GEOLOGY & MINERAL RESOURCES, BULLETIN 160, 2004
239
Age of mineralization in the Luis Lopez manganese
district, Socorro County, New Mexico, as determined by
40Ar/39Ar dating of cryptomelane
Virgil W. Lueth, Richard M. Chamberlin, and Lisa Peters New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801
Abstract
Cryptomelane, K(Mn4+,Mn2+)8O16, samples from the Nancy and MCA mines in the Luis Lopez man­
ganese district were collected in order to determine the age of mineralization using the 40Ar/39Ar dat­
ing method. Cryptomelane precipitation occurred at the onset of calcite mineralization following
banded manganese ore deposition. X­ray diffraction analysis confirmed sample composition and
purity. 40Ar/39Ar dating of two cryptomelane samples from the MCA mine and one from the Nancy
mine yielded age spectra with generally increasing radiogenic yields and apparent ages that suggest
Ar loss. The apparent ages of the oldest furnace analysis heating steps from the MCA­2 sample (6.69
± 0.04 Ma) and 9189 from the Nancy mine (6.33 ± 0.04 Ma) are assigned as minimum ages of mineral­
ization. These ages are consistent with age estimates for mineralization (ca 3–7 Ma) based on geolog­
ic relationships in the area as established by previous workers.
These age determinations indicate manganese mineralization closely followed regional potassic
metasomatism. Published 40Ar/39Ar ages of metasomatic adularia from jasperized fanglomerates
north of the Nancy mine indicate metasomatism persisted until at least 7.4 Ma. Minor manganese
veins near the Nancy mine cut jasperized fanglomerates and cut numerous jasper veinlets in the
Oligocene tuffs affected by this earlier alteration. A published model for the Luis Lopez district previ­
ously interpreted the mineralization to be derived from a manganese­depleted red alteration zone
superimposed on thick intracaldera tuff (31.9 Ma) near the center of the district. A dike dated at 11.05
± 0.05 Ma is unaltered where it cuts across the southern arm of the red zone. This indicates that the
red zone is at least 4.4 m.y. older than manganese mineralization at the MCA and Nancy mines and
genetically unrelated. A composite upper­crustal silicic pluton is inferred to have fed episodic erup­
tions of dacitic and rhyolitic lava domes (at 9.5, 8.7, 7.8, 7.5, and 7.0 Ma) in the Socorro Peak area, 8–9
km northeast of the Nancy mine. This plutonic complex represents the most likely heat source for the
hydrothermal system of the Luis Lopez manganese district. Miocene playa claystones formed an
impermeable cap to the hydrothermal system, thereby forcing significant lateral flow (i.e., 9 km) of
the mineralizing waters. Introduction
Direct dating of hydrothermal ore mineralization has been
the focus of extensive research since the discovery of radio­
metric age dating techniques. However, most deposits con­
tain ore minerals that are not amenable to these methods.
Manganese mineralization in the study area contains cryp­
tomelane, K(Mn4+,Mn2+)8O16, a potassium­bearing phase
that was successfully dated by K­Ar methods as early as
1966 (Chuchrov et al. 1966). Subsequently, the 40Ar/39Ar
technique has been applied to a number of potassium­bear­
ing manganese minerals with varying success (Vasconcelos
1999, 2000). Most geochronological studies have focused on
residual (lateritic) manganese ore deposits rather than
hydrothermal types. To our knowledge this study repre­
sents one of the first attempts at the dating of hydrothermal
manganese minerals.
Mining of vein type hydrothermal manganese mineraliza­
tion in the Luis Lopez manganese district (Fig. 1) began dur­
ing the First World War and continued sporadically until the
early 1970s. Ore was mined from the steeply dipping vein
systems via underground workings and open cuts. Earlier
geological studies in the district were made by Miesch
(1956), Farnham (1961), Hewett (1964), Willard (1973),
Chamberlin (1980), and Eggleston (1982). Fluid inclusion
studies by Norman et al. (1983) detailed hydrothermal char­
acteristics of the district. A study by Eggleston et al. (1983b)
combined alteration patterns with geologic, geochemical,
and oxygen isotope data in the development of a genetic
model for the manganese deposits. Geologic maps of the
Luis Lopez quadrangle (Chamberlin and Eggleston 1996;
Chamberlin et al. 2002) and the Socorro quadrangle
(Chamberlin 1999) provide basic field relationships in the
study area. This information is now augmented by numer­
ous new 40Ar/39Ar age dates on volcanic strata, intrusions,
and potassium metasomatized rocks (cf. Newell et al. 1996;
Dunbar et al. 1996; Chamberlin et al. 2004 this volume). This
paper presents the results of 40Ar/39Ar dating of three cryp­
tomelane samples from Luis Lopez district and compares
these results with data from previous studies.
Geology
Steeply dipping calcite­manganese oxide veins exposed in
the northern and central Chupadera Mountains comprise
the Luis Lopez manganese district located approximately 15
km southwest of the town of Socorro in central New Mexico
(Fig. 1). The Chupadera range represents an east­tilted
extensional fault block uplift of the central Rio Grande rift.
In the Socorro–Magdalena area of the Rio Grande rift, 29
m.y. of progressive crustal extension has broken an
Oligocene caldera cluster and surrounding volcanic plateau
into a north­trending series of tilted­fault­block uplifts and
intervening alluvial basins (Chapin and Cather 1994).
Primary host rocks for north­striking veins of the Luis
Lopez manganese district are densely welded intracaldera
tuffs of late Oligocene age (31.9–27.9 Ma) within the eastern
sector of the Socorro caldera and along the eastern topo­
graphic wall of the younger Sawmill Canyon caldera, which
is partially exposed near the Nancy mine (Chamberlin and
240
FIGURE 1—Series of location maps illustrating pertinent geologic
features within the study area. A–Location of the study area with
respect to the Rio Grande rift (modified from Dunbar et al. 1994).
B–Extent of regional potassium metasomatism (> 6% K2O) derived
from flight lines of the NURE aerial radiometric and magnetic sur­
Eggleston 1996; Chamberlin et al. 2002; Chamberlin et al.
2004 this volume). The northeastern topographic wall of the
Socorro caldera is partially exposed in the west­tilted
Socorro Peak uplift, about 10 km northeast of the Luis Lopez
district.
In middle to late Miocene time, locally derived volcanic­
rich conglomerates and intertonguing playa mudstones
unconformably buried Oligocene volcanic rocks and associ­
ated caldera structures in the Chupadera–Socorro
Mountains area (Cather et al. 1994; Chamberlin 1999). Six
distinct episodes of small volume trachyandesite to rhyolite
lava flows and domes were erupted onto an aggrading
playa floor near Socorro Peak between 9.5 and 6.9 Ma. Four
well­dated episodes of rhyolite extrusion occurred at 8.7,
7.8, 7.5, and 7.0 Ma (Newell et al. 1996). In the Pound Ranch
area approximately 5 km west of the Tower mine, dacite and
rhyolite lava domes were erupted directly onto faulted and
uplifted Oligocene volcanic rocks at about 11.3 Ma (Osburn
et al. 1986; Newell et al. 1996). Similar dacitic lavas (ca 11–12
Ma) locally overlap Miocene conglomerates and Oligocene
tuffs just north of the Tower mine (Chamberlin et al. 2002). vey (U.S. Dept. of Energy 1979a,b). C–Distribution of playa lake
deposits and Oligocene–Miocene silicic volcanic vents in the study
area (from Chamberlin 1999; Chamberlin et al. 2004 this volume).
D–Map of red zone alteration and some manganese deposits in the
study area. (modified from Eggleston et al.1983a).
Faulting is complex and pervasive in the district. Caldera
related faults and north­trending early rift faults have been
rotated about 40–50° to the east in the southern and central
part of the manganese district near the MCA mine
(Chamberlin et al. 2002). Strong rotation of fault blocks in
the rift is attributed to progressive domino­style crustal
extension, which began about 29 m.y. ago (Chamberlin 1983;
Cather et al. 1994). An area of red alteration within the
uplifted and east­tilted caldera­facies Hells Mesa Tuff (Fig.
1) is truncated along its western margin by a large low­angle
rift fault that dips about 30° to the west (Chamberlin et al.
2002 ). Steeply dipping manganese­calcite veins at the MCA
and Nancy/Tower mines appear to post date most of the
fault block rotation that occurred in late Oligocene and
Miocene time. These geologic and structural constraints
originally placed the age of mineralization between 7 and 3
Ma (Chamberlin 1980; Eggleston et al. 1983a). Additional
discussion of the regional structural setting can be found in
Chamberlin et al. (2004 this volume).
No specific intrusive body has been identified in direct
relationship with the formation of the manganese deposits
in the Luis Lopez district. East­trending crystal­poor rhyo­
lite dikes that cut caldera­facies tuff south of the Nancy
mine and a coarsely porphyritic rhyolite plug/dike about 4
km west of the MCA mine were considered to represent pos­
sible heat sources, as well as late Miocene rhyolite domes
near Socorro Peak (Eggleston et al. 1983). Recent 40Ar/39Ar
dating indicates the crystal­poor dikes are of late Oligocene
age (~ 28.7 Ma) and a coarsely porphyritic plug (Chamberlin
et al. 2004 this volume) and an associated dike are of late
Miocene age (11.0 Ma), respectively.
Alteration
Eggleston et al. (1983) recognized two types of alteration in
the Luis Lopez district. They include the southeastern mar­
gin of a regional zone of potassium metasomatism that is
widely exposed in several mountain ranges of the
Socorro–Magdalena area (Fig. 1). A much smaller zone of
“red alteration” confined to lower caldera­facies Hells Mesa
Tuff in the central part of the district (Fig. 1) represents the
second type. A third type of alteration, carbonatization, is
locally associated with manganese oxide mineralization.
Regional potassium metasomatism
Regional potassium metasomatism is of middle to late
Miocene age (Dunbar et al. 1994; Dunbar and Miggins 1996).
Early rift fanglomerates within the broad zone of potassium
metasomatism are an anomalous red color and very well
indurated (Chapin and Lindley 1986). Metasomatism com­
monly extends a kilometer or more into the underlying
Oligocene volcanic pile. Metasomatized rocks are character­
ized by potassium feldspar (mainly adularia) that has pref­
erentially replaced plagioclase; they contain as much as 12
wt% K2O (Chapin and Lindley 1986; Ennis et al. 2000).
Initially, regional potassium metasomatism was interpreted
as the expression of a large geothermal system (D’Andrea­
Dinkelman et al. 1983). More recently, the giant K2O anom­
aly has been attributed to downward percolation of alkaline
basin brines from a large playa system of Miocene age
(Chapin and Lindley 1986; Dunbar et al. 1994).
In the northern Chupadera Mountains fanglomerates of
Miocene age that are potassium metasomatized are also
anomalously well indurated by jasperoidal silica (Cham­
berlin and Eggleston 1996; Ennis 1996). Jasperized fanglom­
erates occur approximately 2 km southwest and 3 km east of
the Nancy mine but do not extend as far south as the MCA
mine. Metasomatic adularia hand picked from clasts in
jasperized conglomerates approximately 3 km north of the
Nancy mine yield 40Ar/39Ar ages of 8.7 ± 0.1 and 7.4 ± 0.1
Ma (Dunbar and Miggins 1996). Minor manganese veins
near the Nancy mine cut jasperized conglomerates and also
cut numerous jasper veinlets in the underlying Oligocene
tuffs. Red zone alteration
Another alteration event, “red zone alteration,” (Fig. 1) was
interpreted to be superimposed on the potassium metaso­
matism zone and directly related to manganese mineraliza­
tion (Eggleston et al. 1983a). Alteration in the red zone is
characterized by the destruction of ferromagnesian minerals
(biotite and hornblende) and subsequent formation of
opaque oxides and sericite. Feldspars are partially altered to
clays and sericite, and the groundmass is reddened by
hematite. Manganese is depleted in the red zone, and by
inference, reconcentrated in the veins around the zone
(Eggleston et al. 1983a). However, manganese mineraliza­
tion is locally present within the red zone along fractures
and faults (e.g., Pretty Girl mine; Willard 1973). Carbonatization
Alteration directly associated with vein and breccia man­
241
ganese mineralization is dominated by calcite. Manganese
oxide and calcite veins consistently crosscut jasperoidal con­
glomerates and veinlets in addition to K­metasomatized
rocks in the district (Chamberlin and Eggleston 1996).
Manganese veins at the north end of the MCA mine cut
Hells Mesa Tuff that contains metasomatic adularia, as doc­
umented by X­ray diffraction analysis (Hewett 1964).
Interestingly, alteration selvages are notably absent adjacent
to mineralized veins in the metasomatized rocks. Calcite,
however, locally replaces plagioclase outside the margins of
veins just south of the MCA mine (Chamberlin and
Eggleston 1996).
Mineralization
Mineralization is hosted by the 31.9 Ma Hells Mesa Tuff at
the MCA mine. The 27.9 Ma Lemitar Tuff and underlying
volcaniclastic rocks of the Sawmill Canyon Formation serve
as host rocks at the Nancy/Tower mines (age data from
Chamberlin et al. 2004 this volume). The form of the
deposits is both as vein breccias and near vertical lenticular
veins in the brittle, densely welded rhyolite tuffs. Ores con­
sist of banded manganese oxides with calcite and variable,
but minor amounts, of quartz (crystal and chalcedony).
Minor manganese­calcite veins at the Nogal Canyon
prospect occur in an 8.4 Ma basalt flow near Walnut Creek,
approximately 4 km south of the MCA mine (Chamberlin et
al. 2002). Gently tilted early Pliocene (?) piedmont­slope
gravels of the lower Sierra Ladrones Formation uncon­
formably overlie late Miocene playa deposits of the
Popotosa Formation (ca 9.5–6.9 Ma) near the west margin of
the Socorro Basin, approximately 4 km east of the Nancy
mine. A 30­cm­thick gravel bed in the lower Sierra Ladrones
near this locality is unusually well cemented by black man­
ganiferous silica and calcite. This minor occurrence may
represent the last gasp of mineralization related to the Luis
Lopez district (Chamberlin and Eggleston 1996).
All ore mineralization in the district occurs as open space
filling. The sequence of manganese oxide precipitation from
the walls to the centers of the veins and breccias varies from
deposit to deposit, even within a single hand specimen
(Willard 1973). Norman et al. (1983) developed a general
paragenesis for the district (Fig. 2). Apparently, deposition
of all manganese ore minerals was essentially simultaneous.
Depositional patterns, reflected by alternating layers of
manganese oxides and calcite, often repeat during the
course of mineralization. Calcite veining tends to post­date
manganese mineralization. Ore mineralization, determined
by X­ray diffraction, consists mainly of cryptomelane­
group minerals: coronadite (PbMn8O16), cryptomelane
[K(Mn4+,Mn2+)8O16], and hollandite (BaMn8O16). Other man­
ganese oxide phases include: pyrolusite (MnO2), and chalco­
phanite ((Zn,Fe,Mn) Mn3O7•3 H2O).
During the course of this study, a particular paragenetic
relation was established for cryptomelane. The most abun­
FIGURE 2—Generalized mineral paragenesis for the Luis Lopez
manganese district as adapted from Norman et al. (1983).
242
70
B.
*
*
*
FIGURE 3—Photographs of mineralization features at the Nancy
mine. A–Sample from the Nancy mine, long dimension is 25 cm.
B–Close­up of the arborescent growths of cryptomelane needles
projecting into calcite.
dant occurrence of cryptomelane is at the manganese oxide­
calcite interface within any manganese oxide­calcite miner­
alization band (Fig. 3). Cryptomelane occurs as dendritic
masses composed of needles projecting into pure, white cal­
cite bands. Apparently, cryptomelane deposition is favored
at the conditions of initial calcite precipitation in these ores.
Needles of cryptomelane were also present in black calcite,
similar to those described by Willard (1973).
Norman et al. (1983) documented the hydrothermal
nature of these deposits by fluid inclusion studies. The tem­
peratures of deposition for the ores are estimated to range
predominantly between 225 and 325°C, based on fluid
inclusion homogenization temperatures. Average homoge­
nization temperatures are generally higher, 275–325°C, in
the northern part of the district compared to those farther
south at the MCA mine, 225–275°C. Most of the fluid inclu­
sions were found to be vapor­rich, indicative of boiling con­
ditions. Freezing determinations on the inclusions indicated
low salinities around 0 wt.% equivalent NaCl. Gas analysis
by Norman et al. (1983) indicated the vapors were dominat­
ed by CO2, and that H2S was absent. Analytical methods and results
Manganese ore samples were collected at the Tower/Nancy
and MCA mines. Mineralogy was determined by a combi­
nation of petrographic analysis and X­ray diffraction.
Standard powder mounts were employed, and X­ray dif­
fraction scans were taken over a range of 2–70º 2θ. Pattern
comparison was accomplished using Jade software and
FIGURE 4—X­ray diffraction spectra for samples analyzed from:
A–the MCA mine and B–the Nancy mine. Standard powder diffrac­
tion file X­ray spectra superimposed on sample pattern.
standard PDF patterns. The resulting X­ray patterns were
determined to be almost pure, crystalline cryptomelane
with little or no contamination by other manganese oxides
(Fig. 4). These patterns are consistent with high K cryptome­
lane described by Vasconcelos (2000), which is highly
amenable to 40Ar/39Ar age dating.
Three cryptomelane samples (MCA­1, MCA­2, and 9189)
were analyzed by the 40Ar/39Ar method using an incremen­
tal­heating, age­spectrum technique. Incremental heating
was achieved with a Mo double­vacuum resistance furnace,
or a 50 watt Synrad CO2 laser using a beam integrator lens.
Samples MCA­2 and 9189 (Nancy mine) were analyzed
using both methods to test sample reproducibility and the
suitability of each method of gas extraction for cryptome­
lane. In addition, sample MCA­2, which initial work had
shown to be fairly well behaved, was vacuum encapsulated
before irradiation to evaluate the potential of 39Ar recoil loss
during irradiation. Following irradiation, the encapsulation
tube was loaded into the CO2 laser chamber, the tube punc­
tured with the laser, and the trapped gas analyzed. The sam­
ple was then removed from the encapsulation tube and ana­
lyzed with the CO2 laser. Abbreviated analytical methods
for the dated samples are given in Table 1, and detailed ana­
lytical data are provided in Table 2.
The six age spectra generated from the Luis Lopez cryp­
tomelane (Figs. 5A–F and Table 2) reveal increasing radi­
TABLE 1—Summary of 40Ar/39Ar data and analytical methods. Sample
Unit/location
Irradiation
Mineral
MCA­2
MCA­1
9189 Nancy
mine
RC­KM­28
MCA mine
MCA mine
Nancy mine
Red Canyon dike
NM­123
NM­123
NM­123
NM­152
** MSWD outside 95% confidence interval
cryptomelane
cryptomelane
cryptomelane
sanidine
243
Analysis
furnace step­heat
furnace step­heat
furnace step­heat
laser total fusion
No. of steps/ MSWD
crystals
14
2.9**
Age
σ
±2σ
Comments
6.69
6.71
6.33
0.04
0.04
0.04
minimum age
minimum age
minimum age
11.05
0.05
Notes:
Sample preparation and irradiation: Mineral separates were prepared using standard crushing, dilute acid treatment, and hand­picking techniques.
Cryptomelane separates were loaded into a machined Al disc and irradiated for 24 hrs in L­67 position, Ford Memorial Reactor, University
of Michigan.
Sanidine was loaded into a machined Al disc and irradiated for 14 hrs in D­3 position, Nuclear Science Center, College Station, TX.
Neutron flux monitor Fish Canyon Tuff sanidine (FC­1). Assigned age = 27.84 Ma (Deino and Potts, 1990) relative to Mmhb­1 at 520.4 Ma
(Samson and Alexander 1987).
Instrumentation: Mass Analyzer Products 215­50 mass spectrometer on line with automated all­metal extraction system.
Cryptomelane separates were step­heated using both a Mo double­vacuum resistance furnace and a 50 watt Synrad CO2 laser using a beam
integrator lens. Heating duration in the furnace 9 min.
Reactive gases removed during furnace analysis by reaction with 3 SAES GP­50 getters, two operated at ~ 450°C and one at 20°C. Gas also
exposed to a W filiment operated at ~ 2000°C.
Heating duration for samples step­heated with a 50 watt CO2 laser using a beam integrator lens was 3 min.
Reactive gases removed by reaction with two SAES GP­50 getters, one operated at ~ 450°C and one at 20°C for 30 or 40 min. Gas also
exposed to a W filament operated at ~ 2000°C and a cold finger operated at – 140°C.
Single crystal sanidine were fused by a 50 watt Synrad CO2 laser.
Reactive gases removed during a 2 min reaction with two SAES GP­50 getters, one operated at ~ 450°C and one at 20°C. Gas also exposed
to a W filament operated at ~ 2000°C and a cold finger operated at –140°C.
Analytical parameters:
Electron multiplier sensitivities: 3.40 x 10­16 moles /pA for furnace step­heat, 1.88 x 10­16 moles/pA for laser step­heat for L#s 50219­02 and
50226­02, 6.99 x 10­17 and 7.10 x 10­17 for L#52167­10, 1.23 x 10­16 for laser total fusion.
Total system blank and background for the furnace averaged 1790, 7.9, 1.5, 1.3, 7.0 x 10­18 moles; 296, 21, 5.1, 0.8, 1.2 x 10­18 moles for the
laser step­heat and 902, 0.64, 0.40, 1.7, and 1.2 x 10­18 moles for laser fusion at masses 40, 39, 38, 37, and 36, respectively.
J­factors determined to a precision of ± 0.1% by CO2 laser­fusion of 4 single crystals from each of 3 or 4 radial positions around the irradi­
ation tray.
Correction factors for interfering nuclear reactions were determined using K­glass and CaF2 and are as follows:
(40Ar/39Ar)K = 0.0225 ± 0.00015; (36Ar/37Ar)Ca = 0.000278 ± 0.000006; and (39Ar/37Ar)Ca = 0.000727 ± 0.000002 for NM­104
(40Ar/39Ar)K = 0.0257 ± 0.0013; (36Ar/37Ar)Ca = 0.00027 ± 0.00002; and (39Ar/37Ar)Ca = 0.0007 ± 0.00005 for NM­136
(40Ar/39Ar)K = 0.0002 ± 0.0003; (36Ar/37Ar)Ca = 0.00028 ± 0.000005; and (39Ar/37Ar)Ca = 0.0007 ± 0 .00002 for NM­152.
Age calculations:
Integrated age and error calculated by weighting individual steps by the fraction of 39Ar released.
Plateau age is inverse­variance­weighted mean of selected steps.
Plateau age is inverse­variance­weighted mean error (Taylor, 1982) times root MSWD where MSWD>1.
Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors.
MSWD values are calculated for n­1 degrees of freedom for weighted mean age.
40Ar/36Ar, and MSWD values calculated from regression results obtained by the methods of York (1969).
If the MSWD is outside the 95% confidence window (cf. Mahon, 1996; Table 1), the error is multiplied by the square root of the MSWD.
Decay constants and isotopic abundances after Steiger and Jäger (1977).
All final errors reported at ±2σ, unless otherwise noted.
ogenic yields and in general increasing apparent ages
throughout most of the 39Ar released. This effect is less pro­
nounced for MCA­2 than it is for either MCA­1 or 9189. It is
also noted that the laser analyses of MCA­2 and 9189 show
the effect to a lesser degree and do not reach as old an appar­
ent age as the furnace analyses. The maximum ages
revealed (except for the highest temperature steps that con­
tain only a few percent of the 39Ar released) by the furnace
analyses of the MCA samples are 6.69 ± 0.04 Ma (MCA­2)
and 6.71 ± 0.04 Ma (MCA­1), whereas that of sample 9189 is
slightly younger, 6.33 ± 0.04 Ma.
The 39Ar recoiled into the encapsulation tube of MCA­2
accounted for 2.7% of the 39Ar released from this sample.
When this gas is added into the integrated age of the stan­
dard step­heat analysis (6.23 ± 0.18 Ma), the resultant age
(5.96 ± 0.30 Ma) is within error. Fourteen single sanidine crystals (RC­KM­28) from a rhy­
olite dike were analyzed by the single­crystal laser fusion
method. A weighted mean age of 11.05 ± 0.05 Ma was calcu­
lated from the 14 crystals (Fig. 6). The age data are displayed
on a probability distribution diagram (Deino and Potts
1990). Abbreviated analytical methods for the dated sam­
ples are given in Table 1. The argon isotopic results are sum­
marized in Table 1, and details of the analysis are listed in
Table 3.
Discussion
40Ar/39Ar cryptomelane dating results
The dated cryptomelane samples exhibit characteristics
desirable for argon age dating; a high degree of crystallinity,
large grain sizes, and a high degree of purity (Vasconcelos
2000). In addition, the small amount of 39Ar released into the
encapsulation tube of sample MCA­2 (< 3% of the total 39Ar
released by this sample) suggests that 39Ar recoil is not a
problem for this type of cryptomelane. However, as men­
tioned above, all of the Luis Lopez cryptomelane age spec­
tra reveal increasing radiogenic yields correlated with
increasing apparent ages that are suggestive of Ar loss. The
apparent ages of the oldest and most radiogenic heating
steps from the furnace analyses have therefore been
244
FIGURE 5—Age spectra from the MCA and Nancy ore. 5A–MCA­2
furnace step­heating analysis; 5B–MCA­2 laser step­heating analy­
sis; 5C–MCA­2 sample vacuum encapsulated before irradiation and
step­heated with laser. 39Ar recoiled into encapsulation tube shown
by unfilled box. 5D–MCA­1 furnace step­heating analysis.
5E–Nancy mine 9189 furnace step­heating analysis. 5F–Nancy mine
9189 laser step­heating analysis. Minimum ages for mineralization
assigned to maximum apparent ages of furnace analyses. * 2σ
assigned as the minimum ages of mineralization for sam­
ples MCA­2, MCA­1, and 9189 (6.69 ± 0.04 Ma, 6.71 ± 0.04
Ma, and 6.33 ± 0.04 Ma, respectively). It is thought that the
laser analyses are more concordant than the furnace analy­
ses due to uneven heating and thus homogenization of the
Ar extracted from the cryptomelane. It is noted that the ages
assigned to the MCA samples agree within error, whereas
the Nancy mine cryptomelane (9189) yielded a slightly
younger age. It is unlikely that these samples have been
reheated to temperatures greater than their temperatures of
formation, 225–325°C. This type of loss spectra has also been
seen in lateritic cryptomelane from West Africa that is also
very unlikely to have undergone reheating (Hénocque et al.
1998). Vasconcelos (2000) has found in thermogravimetric
TABLE 2—Argon isotopic data for furnace and laser step­heating analysis. Isotopic ratios corrected for blank,
radioactive decay, and mass discrimination, not corrected for interfering reactions. Individual analyses show ana­
lytical error only; total gas age errors include error in J and irradiation parameters. n = number of heating steps;
K/Ca = molar ratio calculated from reactor produced 39ArK and 37ArCa; * = 2σ error; D = mass spectrometer discrim­
ination value. Ages in bold text assigned as mininum ages.
40Ar/39Ar 37Ar/39Ar
36Ar/39Ar
39Ar
40Ar*
39Ar
ID
Temp
K/Ca
Age
±1σ
σ
K
(°C)/watts
(x 10­3) (x 10­16 mol) (%)
(%)
(Ma)
(Ma)
MCA­2, 7.16 mg cryptomelane, J = 0.0032679, D = 1.00661, NM­104, Lab# = 50219­01
A
600
373.2
0.0321 1238.3
26.7
15.9
2.0
0.7
B
700
212.3
0.0325
711.5
32.2
15.7
1.0
1.6
C
750
164.1
­0.0046
534.2 9.50 ­
3.8
1.9
D
800
44.22
0.0362
138.5
80.5
14.1
7.4
4.1
E
850
4.643 0.0326
11.08
69.9
15.7
29.2
6.0
F
875
1.666 0.0279
1.851 259.5
18.3
66.9
12.9
G
900
1.232 0.0281
0.5278 570.6
18.2
87.3
28.5
H
925
1.193 0.0284
0.2436 292.3
18.0
94.1
36.4
I
975
1.193 0.0256
0.1230 2252.3
19.9
97.1
97.7
J
1100
2.401 0.0449
0.7208 76.0
11.4
91.3
100.0
total gas age
n=10
3669.5
18.9
42.55
12.29
36.15
19.15
7.94
6.47
6.22
6.48
6.69
12.75
7.41
17.81
10.11
10.09
2.22
0.43
0.11
0.05
0.07
0.02
0.31
0.70*
0.24
2.36
6.11
6.10
6.13
6.29
6.11
6.22
6.55
6.30
6.29
5.97
2.06
1.33
0.09
0.06
0.07
0.07
0.05
0.05
0.06
0.06
0.08
0.30*
MCA­2, 4.86 mg cryptomelane, J = 0.0032679, D = 1.00676, NM­104, Lab# =5 0219­02
A
1
103.7
­0.0148
360.7
4.95
­
­2.8
0.3
B
3
36.13
0.0240
118.9 14.0
21.3 2.8
1.1
C
5
10.66
0.0200
34.01
23.3
25.5
5.6
2.4
D
7
7.274 0.0294
22.78
29.7
17.3 7.3
4.1
E
9
3.981 0.0338
10.40
44.3
15.1
22.5
6.6
F
10
3.106 0.0456
8.779
53.4
11.2 16.1
9.7
G
12
2.230 0.0409
4.276 118.4
12.5
43.1
16.5
H
14
1.605 0.0465
1.790 238.2
11.0
66.9
30.1
I
16
1.460 0.0454
1.298 364.8
11.2 73.7
50.9
J
18
1.429 0.0462
1.172 382.9
11.0
75.8
72.8
K
20
1.406 0.0445
0.9887 313.0
11.5
79.3
90.7
L
22
1.609 0.0485
1.984 149.0
10.5
63.5
99.2
M
24
12.84 0.0970
45.02
13.9
5.3
­3.5
100.0
total gas age
n=13
1749.7
11.6
­16.91
5.91
3.52
3.13
5.25
2.93
5.59
6.23
6.23
6.27
6.45
5.93
­2.66
5.86
MCA­1, 11.25 mg cryptomelane, J = 0.0032665, D = 1.00661, NM­104, Lab# = 50218­01
A
600
2.115 0.0420
5.509 132.0
12.2
22.7
1.4
B
700
1.339 0.0406
2.181 169.7
12.6 51.7
3.2
C
750
0.8097 0.0424
­1.4725 30.6
12.0 156.3
3.4
D
800
1.021 0.0398
0.3578 337.0
12.8
90.1
7.1
E
850
1.128 0.0377
0.2023 1002.7
13.5 95.1
18.6
F
875
1.098 0.0376
0.1174 1073.2
13.6
97.2
30.8
G
925
1.145 0.0365
0.0741 3495.3
14.0 98.4
71.1
H
975
1.186 0.0396
0.0930 2346.8
12.9
98.0
98.1
I
1100
2.775 0.0510
0.4621 151.2
10.0
95.2
100.0
total gas age
n=9
8738.5
13.4
2.80
4.01
7.23
5.29
6.18
6.15
6.49
6.71
15.38
6.50
MCA­2, 14.99 mg cryptomelane, J = 0.0038622, D = 1.0042, NM­136, Lab# = 52167­10
AA
0
57.56
0.1361
195.6
91.7
3.7
0.1
2.7
A
3
35.17
0.0667
118.1
69.2
7.7 1.0
4.7
B
7
2.494 0.0373
5.405 588.3
13.7
35.6
21.8
C
10
1.677 0.0456
2.638 595.6
11.2
53.1
39.1
D
12
1.680 0.0477
2.635 372.7
10.7
53.3
49.9
E
14
1.493 0.0378
1.918 347.8
13.5 61.7
60.0
F
16
1.266 0.0378
1.239 355.3
13.5
70.8
70.3
G
18
1.196 0.0372
0.9466 327.3
13.7
76.4
79.8
H
20
1.292 0.0320
1.110 273.4
16.0 74.4
87.8
I
22
1.258 0.0260
1.113 246.7
19.6
73.5
95.0
J
24
1.359 0.0306
1.460 173.6
16.7 67.9
100.0
total gas age
n=11
3441.5
13.3
9189 Nancy, 15.10 mg cryptomelane, J = 0.0032204, D = 1.00661, NM­104, Lab# = 50227­01
A
600
30.44 0.0418
102.6
567.2
12.2
0.4
5.5
B
700
1.180 0.0362
0.8383 589.5
14.1
80.4
11.2
C
750
1.076 0.0356
­0.3296 54.1
14.3
110.3
11.8
D
800
1.022 0.0369
0.1987 1294.0
13.8
95.3
24.6
E
850
1.089 0.0341
0.1104 1573.0
15.0 97.6
39.9
F
875
1.113 0.0328
0.0937 1092.4
15.6
97.9
50.6
G
925
1.133 0.0327
0.0795 4994.4 15.6
98.2
98.7
H
975
1.056 0.0294
­3.2346 18.3
17.4 193.1
98.9
I
1025
1.384 0.0290
0.7116 89.2
17.6 85.0
99.9
J
1075
2.563 0.0367
­1.0969
8.09
13.9 113.3
100.0
total gas age
n=10
10280.1
15.0
0.69
5.40
6.74
5.52
6.04
6.19
6.33
11.55
6.71
16.64
5.83
8.88
2.68
1.63
1.01
0.49
0.48
0.21
0.11
0.07
0.07
0.08
0.13
1.58
0.42*
0.20
0.14
0.74
0.08
0.03
0.03
0.02
0.02
0.17
0.06*
1.47
0.05
0.32
0.03
0.02
0.03
0.02
0.89
0.19
2.36
0.22*
245
246
TABLE 2—continued.
40Ar/39Ar
ID
Temp
(°C)/watts
40Ar*
39Ar
(%)
(%)
Age
(Ma)
σ
±1σ
(Ma)
9189 Nancy, 3.6 mg cryptomelane, J = 0.0032462, D = 1.00676, NM­104, Lab# = 50226­02
A
3
16.51
0.0372
55.10
166.5
13.7 1.8
6.1
B
7
1.227 0.0288
0.9530 581.4 17.7
78.2
27.5
C
10
1.151 0.0284
0.3715 982.4
18.0
91.0
63.5
D
13
1.170 0.0272
0.3810 761.1
18.8
90.7
91.5
E
16
1.265 0.0269
0.8487 184.4
18.9 80.3
98.2
F
20
1.898 0.0209
3.463 34.0
24.4 45.8
99.5
G
22
4.660 ­0.0184
14.12
9.23
­
10.1
99.8
H
24
11.90 ­0.1336
38.91
5.03
­
3.1
100.0
total gas age
n=8
2724.2
17.9
1.71
5.51
6.00
6.08
5.83
5.02
2.74
2.15
5.62
0.66
0.04
0.03
0.03
0.10
0.54
1.84
3.46
0.18*
37Ar/39Ar
36Ar/39Ar
39Ar
K/Ca
K
(x 10­3) (x 10­16 mol)
and black biotite) where it cuts across
the zone of red alteration in the Hell
Mesa Tuff. Sanidine from this dike is
dated at 11.05 ± 0.05 Ma (Table 3; Fig.
6). A compositionally and texturally
equivalent coarsely porphyritic rhyo­
lite plug that intrudes caldera­facies of
Hells Mesa Tuff near Red Canyon (Fig.
RC­KM­28, single crystal sanidine, J = 0.0014699, D = 1.00712, NM­152, Lab# = 53216
1) yields a precise 40Ar/39Ar age 10.99 ±
02
4.218
0.0057
0.2510
4.46
89.1
98.3
10.96
0.06
0.06 Ma from sanidine phenocrysts
2.847
9.17
73.4
83.2
10.97
0.05
11
4.992
0.0070
(Chamberlin et al. 2004 this volume).
0.1849
9.28
66.3 98.7
10.98
0.04
01
4.207
0.0077
The contact of this nearly contiguous
0.2295
6.26
75.6
98.4
10.98
0.05
09
4.221
0.0067
dike with the plug is obscured by col­
0.1862
9.67
79.6
98.7
11.00
0.03
13
4.214
0.0064
luvium although the age and petrolog­
0.1397
5.27
85.8
99.0
11.00
0.04
07
4.202
0.0059
0.2344
2.76
85.3
98.4
11.02
0.09
14
4.236
0.0060
ic relationships indicate the plug and
0.0655
6.17
72.3
99.6
11.03
0.05
08
4.190
0.0071
dike are part of the same intrusion. The
0.9349
7.54
35.6
93.8
11.04
0.05
03
4.450
0.0143
rhyolite plug occurs in the footwall of
1.492
9.86
58.8 90.5
11.06
0.05
12
4.625
0.0087
the range­bounding fault zone and is
0.1660
5.99
83.6
98.9
11.08
0.04
04
4.239
0.0061
locally cut by manganese veins and
0.6619
12.8
71.4
95.6
11.09
0.03
06
4.391
0.0071
shows evidence of potassium metaso­
0.0260
14.6
85.5 99.8
11.14
0.03
05
4.221
0.0060
matism (Eggleston 1982; Chamberlin
0.1534
7.41
87.5
99.0
11.17
0.04
10
4.272
0.0058
and Eggleston 1996). Fine­grained phe­
75.0 ± 14.4 11.05
0.05*
weighted mean MSWD = 2.9** n=14
nocryst­poor rhyolite dikes of late
Oligocene age (ca. 28.5–28.7 Ma) are potassically altered
studies that poorly crystalline, low K cryptomelane under­
(contain secondary biotite) where they cut red hematized
goes phase transformations and thus possibly Ar loss at
Hells Mesa Tuff south of the Nancy mine (Chamberlin and
much lower temperatures than high K, crystalline cryp­
Eggleston 1996; Chamberlin et al. 2004 this volume). These
tomelane. We suggest that the analyzed samples contain a
observations indicate that the red alteration zone is younger
small percentage of low crystallinity cryptomelane that is at
than 28.5 Ma and older than 11.0 Ma and cannot be the
least partially responsible for the Ar loss that we see. It is
source of manganese mineralization dated at 6.7–6.3 Ma. also possible that the high crystallinity cryptomelane was
precipitated over a period of time, and the gentle increase in
Manganese mineralization and potassium metasomatism
ages seen over the majority of the age spectra actually
The relationship between manganese mineralization and
records the precipitation history of the cryptomelane. The
regional potassium metasomatism remains speculative.
ages determined for these samples are in complete agree­
Although not economically significant, manganese mineral­
ment with field data and isotopic ages that constrain the age
ization in the Magdalena Mountains, west of the Luis Lopez
of mineralization between 7.4 Ma and 3.7 Ma (Chamberlin
district, is nearly as widespread as potassium metasoma­
and Eggleston 1996; Dunbar and Miggins 1996; and
tism and appears to occur in proximity to metasomatized
Chamberlin 1999). rocks (cf. Farnham 1961; Dunbar et al. 1994). Several large
Manganese mineralization and the red zone alteration
areas of potassium metasomatism in the Basin and Range
province are spatially related to Tertiary volcanic centers in
Eggleston et al. (1983b) interpreted the red zone (Fig. 1D) as
strongly extended terranes and Miocene basins (Chapin and
a relatively small epithermal zone locally overprinted on the
Lindley 1986). Manganese districts are also commonly asso­
southeast margin of a broad regional zone of potassium
ciated with these large K2O anomalies in the Basin and
metasomatism. Because the red zone was found to be
Range (Chapin and Glazner 1983; Chapin and Lindley
strongly depleted in manganese and centrally located, it
1986).
was hypothesized as the likely source area for surrounding
Ages of potassium metasomatism in the Socorro region,
epithermal manganese oxide veins that compose the Luis
Lopez district (see Eggleston et al. 1983a; Fig. 5). Recent
as determined by 40Ar/39Ar methods, range from 14.1 Ma at
Water Canyon, 15 km west of the study area, to 7.4 Ma at
mapping and 40Ar/39Ar dating of rhyolite dikes in the dis­
Box Canyon, about 3 km north of the Nancy/Tower mine
trict (Chamberlin and Eggleston 1996; Chamberlin et al.
(Dunbar and Miggins 1996). The minimum age for man­
2002; Chamberlin et al. 2004 this volume) do not support
ganese mineralization in the Luis Lopez district (6.7 Ma) is
this earlier interpretation. slightly younger than local potassium metasomatism (7.4
An east­trending rhyolite dike (RC­KM­28) is distinctly
Ma). These isotopic age relationships are strongly support­
unaltered (contains fresh glassy plagioclase, clear sanidine,
TABLE 3—Argon isotopic results for single crystal sanidine total fusion analysis. Isotopic
ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for
interfering reactions. Individual analyses show analytical error only; weighted mean age
errors include error in J and irradiation parameters. n= number of heating steps; K/Ca =
molar ratio calculated from reactor produced 39ArK and 37ArCa; *= 2σ error; ** MSWD out­
side 95% confidence interval; D = mass spectrometer discrimination value.
40Ar/39Ar
37Ar/39Ar
36Ar/39Ar
39Ar
σ
ID
K/Ca
%40Ar*
Age
± 1 σ
K
(x 10­3) (x 10­15 mol)
(Ma)
(Ma)
247
ed by field observations of manganese
veins that consistently cut metasoma­
tized and jasperized rocks. Geo­
chemical studies have shown that tuffs
and lavas in the Socorro–Magdalena
K2O anomaly are typically depleted in
Na, Ca, Mg, Sr, and Mn (Chapin and
Lindley 1986; Dunbar et al. 1994; Ennis
et al. 2000). Although speculative, the
available data would permit the east­
ern part of the Socorro–Magdalena
K2O anomaly to be a source area for
slightly younger manganese mineral­
ization in the Luis Lopez district. A conceptual model for manganese
mineralization
Timing constraints for manganese
mineralization in the Luis Lopez dis­
trict suggest that upper crustal mag­
matism (as expressed by silicic volcan­
ism), potassium metasomatism and
ore mineralization are interrelated.
Based on these constraints, we propose
the following mineralization model for FIGURE 6—Age probability distribution diagram for sample RC­KM­28 single crystal sani­
dine total fusion analysis. * 2σ ** MSWD outside 95% confidence interval.
the district. Our preliminary model
incorporates many of the features pro­
posed by Eggleston et al. (1983a) and only abandons the
partially assimilated sanidine xenocrysts, represents the last
need for the red zone to be the area for the source of man­
thermal replenishment of this upper crustal magmatic sys­
ganese. Instead, our system is laterally associated with sili­
tem at 6.9 Ma. Without magmatic replenishment, upper­
cic magmatism similar in age to the cryptomelane analyzed
crustal silicic plutons must become completely crystalline
and cooled by hydrothermal circulation within a few hun­
during this study (Fig. 7).
dred thousand years after their emplacement (Lachenbruch
The youngest silicic volcanism in proximity to the study
et al. 1976) Approximately 0.5 m.y. can be subtracted from
area is located west of Socorro Peak and approximately 9
the eruption age of local episodes of silicic volcanism in
km north­northeast of the Nancy mine. The age of this mag­
order to estimate the probable minimum age of each associ­
matism is slightly older than mineralization. Silicic volcan­
ated hydrothermal event (Lipman et al. 1976). Long­lived
ism at Socorro Peak began with eruption of several small
hydrothermal systems at young ignimbrite calderas (> 1
flat­topped dacitic domes (tortas) at 9.5 Ma (Newell 1997;
Ma) suggest that periodic replenishment of crustal magmat­
Chamberlin 1999). Rhyolite lava dome eruptions were then
ic systems is a common phenomenon (Lipman 2000). We
sequentially emplaced in this area at 8.7, 7.8, 7.5, and 7.0 Ma
suggest that an upper crustal composite pluton of granitic
(Newell 1997). Rhyolite volcanism was immediately fol­
composition associated with the Socorro Peak rhyolite lava
lowed by eruption of a xenocrystic trachyandesite lava on
domes is the most likely source of heat energy to drive the
the west flank of the Socorro Mountains (Sedillo Hill) at 6.9
hydrothermal system at Luis Lopez. Ma (McIntosh and Chamberlin unpublished data). This
In our model the Luis Lopez district lies on the outer mar­
extended period of silicic volcanism is interpreted here as
gin of a hydrothermal system centered near Socorro Peak.
the surface expression of the emplacement of a composite
The presence of precious metal sulfide mineralization at the
upper­crustal pluton under the Socorro Peak area in Late
Torrance mine (Socorro Peak district; Lasky 1932) is more
Miocene time. A hybrid trachyandesite magma, containing
FIGURE 7—A schematic model for mineralization and alteration in the Luis Lopez manganese district. Figure based
on diagrammatic cross section of Eggleston et al. (1983b) with modifications from Chamberlin (1999) and
Chamberlin et al. (2002).
248
typical of mineralization proximal to a heat source. Higher
overall fluid inclusion homogenization temperatures at the
Nancy/Tower mines when compared to the MCA mine
(Norman et al. 1983) support a temperature gradient from
north to south. A thick sequence of playa claystones in the
upper Popotosa Formation (Figs. 1,7) essentially serves as
an impermeable cap on the hydrothermal system. As the
hydrothermal fluids emerged from under this cap, boiling
and mineralization occurred. In this scenario the Luis Lopez
manganese deposits represent distal mineralization and are
analogous to argentiferous manganese oxide mineralization
associated with base metal sulfide mineralization in some
manto deposits of northern Mexico (Megaw 1990).
The patterns of mineralization and alteration expected in
our model suggest that potassium metasomatism is a prod­
uct of a hydrothermal system similar to the original inter­
pretation of D’Andrea­Dinkelman et al. (1983). Both man­
ganese mineralization and alteration appear linked to north­
south faults that apparently served as fluid conduits in the
study area, as indicated by the observation that virtually all
bedrock faults northeast of the Nancy mine are filled with
jasperoidal silica (Chamberlin 1999; Chamberlin et al. 2002).
Although older ages for potassium metasomatism have
been determined west of the district (14.1 Ma at Water
Canyon, Dunbar and Miggins 1996), we suggest that the
Water Canyon episode of alteration is associated with an
entirely separate hydrothermal system of middle Miocene
age (cf. Newell 1997). Initial age dating of manganese min­
eralization, similar in character to Luis Lopez, in the south­
ern Magdalena Mountains supports this hypothesis. Studies
are continuing to document these relationships.
Conclusions
The results of 40Ar/39Ar analysis of cryptomelane from two
deposits in the Luis Lopez manganese district agree with
age estimates based on geologic relationships established by
previous workers. Mineralization in the entire district is
nearly contemporaneous based on the similarity of ages,
coupled with the wide separation (geographic and strati­
graphic) of the MCA (6.69 ± 0.04 Ma, minimum age) and the
Nancy (6.33 ± 0.04 Ma, minimum age) mines. The timing of
mineralization is coincident with the waning of upper
crustal silicic magmatism in the Socorro Peak area (8.7–7.0
Ma), which represents the most likely heat source for the
hydrothermal system. Timing relationships with respect to
the regional potassic metasomatism indicate the manganese
mineralization (~ 6.7 Ma) is slightly younger than metaso­
matism (7.4 Ma; Dunbar and Miggins 1996) although they
may be interrelated.
Acknowledgments
We would like to thank Drs. Andrew Campbell, William
McIntosh, and Matthew Heizler for their constructive
reviews of the manuscript. Richard Esser provided assis­
tance with the Ar analysis. Chris McKee assisted with the X­
ray diffraction analysis. The New Mexico Bureau of Geology
and Mineral Resources, Dr. Peter Scholle, Director and State
Geologist, supported this work. References
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